Electrical measuring device



June 30, 19 42.

):'o 40 a; so no I20 I40 no nozmz'zqnoz'w A.C. MILLIVOLTS (man) 5. C.HOARE EI'AL ELECTRICAL MEASURING DEVICE Filed Oct. 11', 1940 Fig.5.

' an m a V muss w'ii LEFT PERCENT CHANGE IN BRIDGE ARM Inventors;Stephen C. Hoare,

Allen T. Sipks,

i! s X I b WW 6 32 y Their-Atiaorney Patented June so, 1942ansc'rnrcanmmsuamg nnvrca Stephen C. Hoar-e, Manchester, and Allen '1.

Beach Bluff, Mass. ,tric Company,

assignors to General Eleca corporation of New York Application October11, 1940, Serial No. 360,782

7 Claims.

This invention relates to electrical measuring apparatus and moreparticularly to electrical apparatus of the type employing rectifiers inconjunction with a device which is responsive to direct current or adirect current measuring instrument for measuring alternating currentquantities.

It is common in the art of electrical measure- .ments to measurealternating current voltages or currents by means of a direct currentinstrument in conjunction with one or more rectifiers. This method iswell suited to the measurement of values of comparatively largemagnitude. On the other hand, the foregoing method, though it issuperior to the conventional type alternating current meter, is stillnot sufiicient- 1y accurate, reliable or sensitive in the lower range ofcurrent values where accurate indications. of output current are oftendesired. Consequently, where small values of alternating voltages orcurrent are under observation, the divisions on the scale of theindicating instrument will be quite crowded. The foregoing disadvantagesare attributed to the fact that certain rectifying elements, such forexamples as diode valves of the thermionic type and copper-oxiderectifiers of the contact type, exhibit-a very high resistance to thepassing of current in the lower part of their operating characteristicor voltage range. Consequently, the slope of the characteristic curveshowing the relation between impressed potential and output current isso small at or near the bottom portion of the curve that the outputcurrents corresponding to minute impressed potentials are too small toproduce a sensitive deflection or movement of the direct currentresponsive instrument. This resistance, however, decreases materiallywith increased magnitudes of potential applied to the circuit of therectifiers.

It is an object of our invention, therefore, to provide an improvedmeasuring device which is particularly adapted to the measurement ofminute alternating current quantities.

It is another object of our invention to improve the eiiiciency andsensitivity of a rectifiertype measuring instrument.

It is a further object of our invention to provide in a rectifier-typemeasuring device a more linear scale distribution in the lower range ofmeasurement.

It is another object of our invention toprovide an improved biasingarrangement for a rectifiertype measuring instrument which permits therectifier elements to be biased to the desired through the rectifier byalternating current quantities of small mag-.

nitude are rendered efi'ective to produce an appreciable deflection orresponse in a direct current responsive device or instrument. Theforegoing beneficial result is accomplished by the use of a novelcircuit arrangement which is adapted to be connected to the alternatingcur- 'rent circuit under observation and which comprises a plurality ofrectifiers, a source of direct current potential for biasing therectifiers, and a direct current instrument, preferably of the dArsonvaltype. The circuit may be controlled to pass the desired amount ofbiasing current element and it may be further adjusted so that a desiredportion of this biasing current fiows through the current coil of themeasuring device in either direction or so that none of the biasingcurrent fiows through the measuring element. By means of the foregoingarrangement, we have been able to increase the accuracy, eificiency andsensitivity of a direct current instrument when employed in conjunctionwith a rectifier for the detection and measurement of small alternatingcurrents or voltages near the zero level and, in the particular case, ofalternating current voltages or our rents below the range of sensitivityof the usual alternating current instrument or current responsivedevices.

The novel features which we believe to be characteristic of ourinvention are set forth with particularity in the appended claims. Ourinvention itself, however, both as to its organization and method ofoperation may best be understood by reference to the followingdescription taken in connection with the accompanying drawing.

In the drawing, Fig. l is a diagrammatic representation of arectifier-type measuring instrument arranged in accordance with ourinvention; Fig. 2 is a graphical view showing the characteristics of aconventional rectifier-type measuring instrument, and the correspondingcharacteristics of a rectifier-type measuring instrument in corporatingthe principles of our invention and of the character shown in Fig. 1;Figs. 3 and 5 illustrate the scale distribution of conventionalrectifier-type measuring instruments; Figs. 4 and 6 are views showinghow the respective scales of Figs. 3 and may be expanded in the lowerrange by the employment of rectifier-type measuring instruments arrangedin accordance with our invention; Fig. 7 is a differential typemeasuring device in which the principles of our invention are employed;and Fig. 8 is a graphical view showing characteristics obtained by theuse of the arrangement of Fig. 'l in contrast with those obtained by aconventional instrument.

Referring now to the drawing, in Fig. 1,, we have shown a rectifier-typemeasuring instrument in. the form of a bridge and which may comprise tworectifier elements such as it and ill or l2 and 63 in adjacent arms andtwo impedance elements in the remaining adjacent arms or which may bearranged with a rectifier element in each arm as shown in the drawing.

The rectifier elements til, it, it and it, which are connected in therespective arms of the bridge and the impedances of which decrease asthe applied voltage increases, may be of any suitable construction, suchfor examples as a 1 metal or copper-oxide rectifier or a thermionicvalve of the diode type. The alternating current quantity, such as acurrent or voltage to be measurcd. is connected to the input terminals95 and [16 of the bridge. To the output terminals ill and it of thebridge is connected the current coil it of a direct current responsivedevice 2@, such as a dArsonval measuring instrument.

With a circuit of the character thus far described, if values of theindicated direct current intensity as measured by the instrument iscorresponding to a number of different values of input voltage beplotted in graphical form a characteristic curve will be obtained,similar to curve A of Fig. 2. The slope of the curve is a function ofthe overall admittance of the rectiiier elements and the coil it? of themeasuring device. An inspection of curve A indicates that within thelower range of voltage applied to the input terminals 55 and it thecurve tends to parallel the abscissa so that relatively large variations in the applied voltage are necessary to efiect minute changes ofthe output or direct current flowing from the terminals ii and it. Onthe other hand, within the range of comparatively high applied voltagesthe slope of the characteristic curve becomes steeper so that a givenvariation in applied voltage is accompanied by current flow in theoutput circuit which is greatly accentuated over the current flow whichthe same variation in applied voltage would produce with operation onthe lower portion of the characteristic curve of the rectifier. Curve B,when contrasted with curve A, illustrates the marked increase insensitivity obtained by the employment of our invention.

Reference to Fig. 3, which shows a scale 28 calibrated in alternatingcurrent millivolts, indicates that a rectifier-type instrument gives avery crowded scale when operated in the lower portion of thecharacteristic curve of the rectifier and is thus relatively insensitiveto the values of applied potential within the vicinity of the zerolevel.

We provide means for biasing each rectifier element so that it operatesin the steep part of its input-output characteristic even for minutevalues of the applied potential or current to be measured. In theillustrated arrangement, an adjustable resistance element 22 of asuitable aasaaee magnitude determined by the other constants of thecircuit is connected in series circuit relation with the coil 3 9 of thedirect current measuring instrument to the output terminals ill and i8oi. the rectifier bridge. This resistance element may be made adjustableor fixed depending upon the method to be employed in adjusting thesystem. This feature will be explained more fully hereinafter. Aresistor in the form of a potentiometer having an adjustable contact 2 3is connected in shunt with the coil is of the current responsive deviceThe contact 25 divides the resistor 23 into complementary portions 25and A source of unidirectional potential 28 such as a dry cell isconnected between the point 58 and the adjustable contact 241 in seriescircuit relation with an adjustable resistance element 29.

in Fig. 4 we have illustrated diagrammatically a direct currentinstrument at of the d'Arsonval type with its movable coil 89 secured toa rotatable spindle or shaft Eli and positioned to interact with thefleld produced by permanent magnets iii. The spindle to is shown mountedin suitable jewels 32 and is arranged to operate a pointer 33 whichcooperates with a scale 23a. In the instant case the scale is calibratedin alternating current millivolts. Electrical current from the directcurrent output terminals ii and it may be conducted to the current coil69 through a pair oi control springs such as 341. One end of each spring36 is secured to the shaft tit and the other end is secured to aconnector element 35 by means of which the control spring may beadjusted to position the'pointer 33 at any desired angular positionalong the scale 25a. in the arrangement oi Fig. 4 the natural mechanicalzero position of the pointer as determined by the adjustment or thecontrol spring it, is set at anoint approximately 35 degrees up-scaleand the electrical zero is set at the conventional left-end position.

The arrangement oi the present invention includes means whereby eachrectifier element is biased for a condition of zero input at themeasuring terminals iii and iii, the biasing being adjusied for the mostsensitive scale range that starts from zero since this is what isrequired of a straight defiectional instrument that measures values downto zero. Thus, when the measuring device is arranged to operate as shownin Fig. i with the electrical zero out of coincidence with themechanical zero, the system of Fig. i is initially adjusted so that aresultant current flows through the coil it? from the source 2t in theproper direction to cause the pointer 33 to move from position to thezero-point on the scale 2811 as shown in the drawing. From an inspectionof Fig. 1 it will be observed that with the rectifiers poled as shown,the measured current flows from a to b in the coil it to produce anup-scale deflection of the pointer Thus to cause the pointer to movedown scale to the electrical zero position the circuit is initiallyadjusted with zero input at the measuring terminals l5 and it so that aresultant current from the source it flows through the coil 89 from b to(1.

Considering the system connected as in Fig. i and with zero input at themeasuring points i5 and it. by adjusting the potentiometer contact allor the resistor 22 the desired amount of current may be made to traversethe coil iii of the instrument it from the battery 28 and by adjustingthe resistor is the proper amount of biasing current may be sent throughthe rectifier aasasao elements to give the most sensitive response. Oncethe potentiometer contact 24 or the resistance element 22 has beenset togive a zero scale indication for the proper biasing current, afterproper calibration, the device is ready for use. Thereafter, a preciseand predetermined degree 01 biasing is applied simply by adjusting therheostat 28 until the pointer SI swings from a mechanical to anelectrical zero point on the instrument scale. The instrument is thusbrought into a precalibrated condition without the need of using extrainstruments to set the degree of biasing.

It will be appreciated that the connection 2 may be permanently made toan intermediate point on the coil I, it desired, in which case thepotentiometer which shunts the instrument may be dispensed with. Theresistor 22 may then be employed to initially adjust the instrumentzero.

However, the potentiometric arrangement provides greater flexibility andit is of more simple construction since it is not necessary to make aconnection to the coil turns. The potentiometer permits oi'by-passingrelatively heavy biasing current around the very sensitive instrumentwithout aflecting its state or zero indication, and it further permitsoi abstracting, lust enough of the heavy biasing current to enectcontrol or the zero or reference position or the instrument. Toaccomplish the iiasing without the use or a potentiometer wou requirethe use of a relatively high current element for the instrument in orderto take care of the large value of biasing current required. Theinstrument would in consequence be rendered relatively insensitive tothe small quantities under measurement. It should also be pointed outthat the contact 24 may be permanently made at point b to apply thebiasing potential directly across the resistance element 22. In such acase, however, the bias point may be only at some up-scale positionwhereas the illustrated embodiment permits adjustment oi the bias pointeither up or down the scale as desired.

In connection with the operation of miliivoltmeters to measureelectrical quantities of small magnitude it is customary to employ aswamping" resistance to improve the. temperature characteristics. InFig. 1 such a resistance is indicated at 37 and is connected in serieswith the rectifier input. By the insertion of an additional resistanceelement such as 38, the instrument may be employed as a voltmeterwithout biasing by dispensing with the dry cell or source of potential28 and the small rheostat 29.

The description of the arrangement 01' Fig. 1 has thus far beenpredicted on the condition of operation, as illustrated in Fig. 4, inwhich the mechanical and electrical zeros are positioned at somedistance apart on the scale in order to enable the user to make a morepositive adjustment of the bias, that is, to the electrical zero point.Ii. preferred, however, the two zero positions may be made coincidental,the user then turning the dication before applyin the voltage to bemeasured. It will be apparent that such a condition of operation isobtained when the potential drops across sections 25 and 28 are equaland opposite so that for zero input to the measuring terminals II and I8none of the current from the source 28 flows through the instrument coill9. Fig. 6 illustrates the scale characteristics obtained with a 200milliampere capacity millivoltmeter when arranged as in Fig. 1 buthaving coincident merheostat 29 to maintain the zero incircuitconditions lie.

chanical and electrical zeros. The scale distribution of Fig. 5 wasobtained with the same instrument connected without biasing in aconventional circuit. The marked diflerence in the two cases in themagnitude or the current required for full scale deflection is apparentfrom inspection or the two scales. When the system is adapted for use asa dual instrument, that is as a combination A. C.millivoltmeter-voltmeter, it may be found preferable from the standpointof avoiding confusion to have coincident electrical and mechanical zerosso that the zero value of each scale lies at the same position on thescale. It may be found desirable, however. to eii'ect a compromise inthis respect by using a very small oflset in the zeros so that theoperator may make certain that the biasing system is When thearrangement is operated withcoincident zeros the potentiometer in shuntwith the coil is must initially be unbalanced so that the operator mayapprise himself as to the amount oi biasing potential being used. I

Irrespective of whether the foregoing system is operated with adisplacement or coincidence between the mechanical and electrical zeros,the ectifier is biased so that it carries a current somewhere nearnormal current density or to a value sufllcient to give the mostsensitive scale range that starts from zero. The sensitivity 01' theinstrument in the lower range has been greatly increased because oi thefact that the biasing arrangement reduces the input impedance so thatthe instrument draws more current and causes the rectifier to operate ata more efficient part of the characteristic curve. Consequently, it ispossible to obtain relatively large indications with low orders ofvoltages-voltages too small to produce appreciable indications in aninstrument as usually constructed. The biasing means of the presentinvention also tends toward more linear scale distribution.

In Fig. 7 we have illustrated the principles of our invention as appliedto a diilerential rectifier sible to ascertain on which Referring toFig. 7 sh wn a perable.

current-responsive element No is connected between balance points (iiand 18 in series 'With current limiting impedances Q9, 56 and rectifiers5i and 52. The rectifiers 5i and 52 are so arranged that one tends topermit current to flow in only one direction and the other tends topermit current to fiow in only the other direction with respect to thebalance points. For example, the rectifiers 5! and 52 may be arranged sothat current tends to flow only toward the balance points ill and 38 oronly toward the currentresponsive device l9a as shown in the drawing.

In order that the direction of deflection of the current-responsivedevice will indicate in which arm ll or 2, for example, there is thegreater voltage drop-assuming these to be fixed impedances and arm All],for example, the element of which the impedance is to be measured-acommon terminal of the parallel circuits, such as the terminal 85, isconnected by means of a conductor 53 to a neutral point $5311. withrespect to the ends of the winding of the current-responsive device lea.This may be done by dividing the winding of the current responsivedevice into two portions with a common terminal forming a neutral pointas shown in Fig. 7a so that a null deflection is obtained when thecurrents or potential drops are equal, or it may be done as shown inFig. '7 by connecting an impedance 23a in shunt with respect to the endsof the simple single winding of the current-responsive device 9% andjoining the point 55 With a point 53a, which is preferably the midpointof the impedance 23a. The impedance is shown divided into sections 53and 55.

In order to increase the sensitivity of the cur rent-responsive devicelea we connect a suitable source of direct current, such as a batteryHit, in the line and between points and 53a. The source of supply 5b ispoled to send current through the rectifier elements as shown by thearrows and is of sufiicient voltage to pass the desired biasing current,for example in a particular case, five milliamperes for the rectifierelements, through the resistance or impedance of the bridge arms. Itwill be appreciated that for equal biasing potentials across thesections 56 and 55 there will be a null deflection of thecurrent-responsive element lilo.

In the operation of the apparatus of Fig. 7, assuming a symmetricalarrangement of resistors in the galvanometer circuit, where it and 5d,are of equal magnitude, and also where the potential drops across Bi and62 are equal as obtains with a balanced condition of the bridge, theinsertion of the battery 56 generally has the effect of throwing thecurrent-responsive device off balance in a degree determined by thedissimilarity of the two half-wave rectifiers 5i and 52. Galvanometerbalance can then be restored by altering resistors 39 and 58. For thisreason it is preferable to make one of these resistors, such as 59, inthe form of an adjustable rheostat. The galvanometer may also be broughtinto balance by making the contact 53a adjustable as illustrated by thecontact 2 5 of Fig. 1. Current fiowing through the rectifier element 5iand impedance i9 divides, part flowing through the upper portion 5d ofthe potentiometer or impedance 23a and the other part flowing throughthe currentresponsive device tea and the lower portion 55 of theimpedance 23a to the intermediate point 5311. Current flowing throughthe impedance 5f: and the rectifier 52 will divide in a similar mannerbetween the currentresponsive device 1911 and the lower portion 55 ofthe impedance 23a, flowing, however, in the opposite direction throughthe current-responsive device 59a; It will be appreciated that in thecase of directcurrent measurements, the polarity of the source it willbe made such that-the point M is positive and the point 515 is negative.

The deflection of the current-responsive device E90. will be dependentupon the algebraic sum of the two currents flowing through theimpedances 39 and 58. If the value of one of the impedances of thebridge is then varied so as to throw the circuit into unbalance in theopposite direction, current in the current-responsive device i911 willbe in the opposite direction and a deflection in the opposite directionwill be produced, thereby indicating that the point of balance has beenpassed in changing the impedances of the bridge. Obviously measurementsmay be made either by adjusting the impedances until a balance isobtained or by retaining a fixed adjustment and calibrating theinstrument Na in terms of the quantity to be measured.

It will be apparent that the response of the device lilo depends uponthe difference in absolute values of the voltages in arms ii and G2 andis independent of phase relationship where alternating current impedancemeasurements are being made. Thus by omitting elements iii and lit, thedifferential measuring system may also be employed for comparingalternating or direct currents or voltages in any two electricalcircuits, as well as in bridge circuits of the general character shownin the drawing, merely by producing potential drops in the arms [it and3! which are proportional respectively to the voltages or currents beingcompared. Where the arms ll and 32 are energized by direct current, carewill obviously be taken to connect them with the proper polarities withrespect to the measuring apparatus. The instrument i911 will thusrespond to either alternating or direct currents and it will alsoindicate which of two opposing circuits is carrying the greater current.

In Fig. 8 we have illustrated characteristic curves showing the relationbetween the current in the galvanometer coil lQa or the deflection ofthe movable element with various degrees of change in one of the arms ofthe bridge in Fig. 7 such as arm Ali-i. The curves 58 and 59 illustratethe results obtained by the omission of the energy source 56respectively for increasing and decreasing values of impedance in armd3, while curves tit and iii illustrate corresponding results obtainedwith the circuit as shown in Fig. 7 respectively for increasing anddecreasing values of impedance in the arm it. It is apparent that thegain in sensitivity due to biasing is approxi mately three to one.

In accordance with the provisions of the patent statutes we havedescribed the principles of operation of our invention together with theapparatus which we now consider to represent the best embodimentsthereof, but we desire to have it understood that the apparatus shown isonly illustrative and that the invention may be carried out by othermeans,

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. In an electrical network responsive to an electrical quantity, inputterminals for impressing said quantity on said network, a rectifier element, a direct current-responsive device including a current conductingwinding, means connecting said rectifier element and said Winding inseries circuit relation with respect to each other and in circuit withsaid input terminals, an impedance element connected in parallel withsaid winding, said impedance element having a terminal intermediate theends thereof, an auxiliary source of unidirectional potential, and meansconnecting said auxiliary source of potential in a circuit includingsaid intermediate terminal and said rectifier element for causing anauxiliary biasing current to flow through said rectifier element inorder to increase the sensitivity of said current responsive device toelectric quantities connected to said input terminals.

2. An electrical measuring apparatus comprising a direct-currentresponsive device having a. pair of main terminals and a pair ofrectifiers each having a pair of terminals, one of the terminals of oneof said re'ctifiers being connected to one terminal of saidcurrent-responsive device, one of the terminals of the other of saidrectifiers being connected to the other terminal of saidcurrent-responsive device, the rectifiers being arranged for currentpassage in opposite directions, said current-responsive device having aterminal intermediate to said main terminals, impedance means connectedin shunt with the circuit including said current responsive device andsaid rectifiers, said latter impedance having an intermediate pointbetween which and the end terminals thereof may be impressed electricpotentials to be compared, an auxiliary source of unidirectionalpotential, and means connecting said source of potential between theintermediate terminal of said current responsive device and theintermediate point of said impedance means, said source of potentialbeing of such polarity as to cause a biasing current to flow throughsaid rectifier elements to increase the sensitivity of saidcurrent-responsive device to the potentials impressed on said impedancemeans.

3.. An electrical apparatus for comparison measurements of twoelectrical quantities, a pair of impedance elements connected to have acommonterminal and independent terminals, said impedance elementsadapted to be'subjected respectively to said electrical quantities andin such a way that each of said independent terminals are of the samepolarity and of opposite polarity to said common terminal, adirect-current responsive electrical device having a pair of terminals,a pair of half-wave rectifiers, one of said rectifiers being connectedbetween one of the terminals of said current-responsive device and oneof said independent terminals, the other of said rectifiers beingconnected between the remaining terminal of said current-responsivedevice and the other of said independent terminals, said rectifiersbeing arranged for current passage in opposite directions with respectto said current-responsive device, an impedance connected in parallelwith said current-responsive device, an auxiliary source ofunidirectional voltage, and means connecting said auxiliary source ofvoltage between the common terminal of said impedance elements and apoint intermediate the ends of the impedance which is connected inparallel with said current-responsive device, said source of voltagebeing of such polarity as to cause current to flow therefrom throughsaid rectifiers.

4. In electrical measuring apparatus, a full wave rectifier comprisingfour rectifier elements connected in a bridge circuit, input terminalsand direct current output terminals for said bridge, a direct-currentresponsive device connected between saidoutput terminals, apotentiometer connected in parallel with said currentresponsive device,said potentiometer having a contact adjustable to different positionsintermediate the ends thereof, an auxiliary source of direct currentpotential, and means connecting said source of potential between saidadjustable contact and one of said output terminals, the polarity ofsaid source being such that a biasing current is caused to flow througheach of said rectifier elements for decreasing the forward impedance ofsaid elements to improve the sensitivity of said current-responsivedevice.

5. In rectifier-type measuring apparatus, a full-wave rectifier having apair of input and out-, put terminals, a current-responsive devicehaving a current conducting winding, an adjustable resistanceelement'means connecting said winding and said resistance element inseries between. said output terminals, an impedance device connected inshunt with said winding, an auxiliary source of unidirectional voltage,and means connecting said auxiliary voltage source in shunt with saidadjustable resistance element and a portion of said impedance device,the polarity of said auxiliary voltage source being arranged to send abiasing current through the rectifier elements comprising said full waverectifier for changing their impedance characteristic with respect toelectrical quantities to be connected to said input terminals in orderto increase the sensitivity of said current-responsive device.

6. In a bridge circuit having a pair of opposite terminals forconnection to a current source and asecond pair of opposite terminalsfor connection to a current responsive device, differential indicatingmeans comprising a direct-cunrent responsive device, a pair of half-waverectiflers arranged for current passage in opposite directions andconnected on either side of said current responsive device in seriestherewith between the second-mentioned pair of terminals of said bridgecircuit, an impedance connected in. parallel with said direct-currentresponsive device, an auxiliary source of unidirectional potential, andmeans connecting said source of unidirectional potential between anintermediate point of said impedance element and one of the currentsupply terminals of said bridge circuit, the polarity of said auxiliarypotential source being arranged to send current through said rectifiersin the forward direction.

7. An electrical measuring network, input terminals for impressing avoltage on said network, rectifier means, a direct current responsivedevice including a current conducting winding, means connecting saidrectifier means and said winding in series circuit relation with respectto each other in said network, an impedance element connected inparallel with said winding, said impedance element having a terminalintermediate the ends thereof, an auxiliary source of uni-directionalpotential, and means connecting said auxiliary source of potential in acircuit including said intermediate terminal and said rectifier meansfor causing an auxiliary biasing current which flows through saidrectifier means in order to increase the sensitivity of the directcurrent re sponsive device.

STEPHEN C. HOARE.

ALLEN T. SINKS.

